Background sessions do not support uploads from data for obvious reasons. Solution was to add a check to the encoding logic to check whether the session was configuration as a background session. Also added tests to verify the logic now works without crashing.

# Conflicts:
#	Alamofire.podspec
#	CHANGELOG.md
#	Source/Info.plist

In README.md section "Creating a Manager with Background Configuration": replaced reference to deprecated method backgroundSessionConfiguration of NSURLSessionConfiguration with the new method as of iOS 8.0 called backgroundSessionConfigurationWithIdentifier

Removed extra line

Prevent unnecessary call to the response serialiser in case of an error picked up by the TaskDelegate.

This reverts commit 30a55d6c.